Piston rings are well known. They are usually received within an annular groove disposed about an outer periphery of a piston. In turn, the piston is reciprocated within a cylinder of an internal combustion engine or a compressor. The piston compresses fluids such as gases within a chamber of the cylinder. In the case of an internal combustion engine, these fluids are ignited and expand within a combustion chamber, forcing the piston away from the point of ignition. In the case of a compressor, these fluids are compressed within a compression chamber.
The primary function of piston rings is to provide an effective seal of the gases, called "blow-by control", and is accomplished by placing a first piston ring, called a compression seal, near the uppermost portion of the piston. The compression seal is designed to seal during an up-stroke of the piston to compress the gases within the chamber. A secondary function of piston rings is to prevent excess lubricating oil from entering the chamber. To accomplish the secondary function, a second piston ring, called an oil seal, is placed below the compression seal to prevent oil from being carried up into the chamber. Thus, the compression seal and the oil seal both cooperate to effectively seal the chamber from escaping gases or entering lubricating oil.
Typically, a piston ring comprising the oil seal is discontinuous, having two end portions. The end portions are separated from one another, forming a gap, to expand the piston ring for insertion into a corresponding groove of the piston. The piston ring is then compressed, bringing the end portions closer together, to install the piston within the cylinder. Once installed, a gap still exists between the two end portions of the piston ring. As a result, single piece oil seals are ineffective because oil may pass through the gap between the two end portions.
Oil seals therefore typically include multiple pieces, and the design of oil seals plays an important role in sealing. One known oil seal includes three pieces for oil control, including first and second piston rings, or rails, separated by a spacer. The first and second rails cooperate to scrape oil from an inner surface of the cylinder while the spacer maintains a distance between the first and second rails to prevent overlap or interference between the rails. In another known arrangement, an expander ring is used to bias first and second rails outwardly towards a cylinder wall. It is also known to provide first and second rails separated by a resilient spacer that includes a circumferential slot for receiving a third rail. In such an arrangement, the center rail is usually allowed to float within a space provided between the two outer rails, or else the center rail is secured in place and the two outer rails are allowed to freely move. Any of the known three rail arrangements raises the possibility of excess movement by a rail such that it interferes with proper operation of the remaining rails. Further, because each rail includes gapped ends, oil may leak past the oil seal through the combined gapped ends and make its way into the upper portion of the cylinder.